Another 23 cases that could not be confirmed in the lab were designated as clinically compatible, the researchers reported in the Nov. 21 issue of the New England Journal of Medicine.

Investigation also uncovered wild-type virus in 14 of 673 healthy contacts of patients with acute flaccid paralysis and in 13 of 491 healthy people who had no contact with cases.

The outbreak was a surprise since China has been certified polio-free since 2000, the investigators noted.

But genetic analysis suggested that the virus had been imported from Pakistan, although the index case -- a 16-month-old girl -- belonged to a family with no history of travel outside of Xinjiang.

The investigators were unable to find any direct epidemiological link to Pakistan.

The report comes in the context of increasing numbers of polio cases reported in areas previously free of the disease.

"We have three countries that are left that have never been polio-free -- Nigeria in Africa and Pakistan and Afghanistan in the Middle East," commented William Schaffner, MD, of Vanderbilt University in Nashville.

"They're now exporters of the polio virus," he told MedPage Today.

The most dramatic situation, Schaffner said, is Syria, where civil war has disrupted public health and created thousands of refugees.

Meanwhile, Nigeria, Pakistan, and Afghanistan are reporting a combined total of 119 cases so far this year, down from the 182 the three nations had this time in 2012.

Humans are the only host for the polio virus, so it's theoretically possible to eradicate it. And indeed, the number of cases worldwide has fallen dramatically -- from an estimated 400,000 in 1980 to just 334 so far this year.

That's a global success story for eradication experts, but worryingly for preventive medicine specialists, that total is a third higher than the 223 cases seen in all of 2012.

"Here in the U.S., we're polio-free," Schaffner said. But that status is built on making sure that everybody gets vaccinated. "It's terribly important," he said.

Poliomyelitis "doesn't need a passport," he said -- it can be carried in the intestines and make its way into sewage and water systems.

But if the virus has nowhere to go -- if everyone is immune -- outbreaks can't happen, Schaffner said.

It's low immunization rates, often a consequence of political turmoil or religious beliefs, that are harbingers of more outbreaks, he said.

In the Middle East, preventing potential outbreaks will require vaccinating or re-vaccinating millions of people, many of them children, in countries with various levels of civil upheaval.

"It's a huge challenge," Schaffner said.

In the Chinese outbreak, officials did not have to contend with civil strife, but they did find that their immunization rates had been suboptimal.

In a small study done before they rolled out supplementary immunization campaigns, they found that, overall, 90.4% of participants had antibodies against polio, with geometric mean titers averaging 1:39.

But among children younger than 5, the rate was between 80% and 92%, and the antibody geometric mean titers were 1:40 to 1:106 -- "considered to be relatively low," the researchers wrote.

The index case was reported on July 5, 2011, Yang and colleagues wrote, and the outbreak was confirmed Aug. 26.

More than 1,000 health workers were trained in response immunization and surveillance by Aug. 31, and more than 5 million doses of trivalent oral polio vaccine had been shipped to Xinjiang by Sept. 1, with the first vaccination under way by Sept. 8.

All told, 43 million doses of vaccine were administered in five rounds of vaccination, three with the trivalent vaccine and two with a monovalent vaccine against poliomyelitis type 1.

There were no new cases after Oct. 9, 2011 -- a month and a half after the outbreak was confirmed, Yang and colleagues reported.

The response was "timely, precise, vigorous, and successful," Schaffner said.

But the outbreak showed, Yang and colleagues concluded, that "until wild-type poliovirus transmission is interrupted globally, poliomyelitis-free countries will continue to be at risk for viral importation."

The analysis was supported by the Chinese CDC. The authors did not report any financial links with industry.

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